MBR Booting from DOS
- by eflukx
For a project I would like to invoke the MBR on the first harddisk directly from DOS. I've written a small assembler program that loads the MBR in memory at 0:7c00h an does a far jump to it. I've put my util on a bootable floppy. The disk (HD0, 0x80) i'm trying to boot has a TrueCrypt boot loader on it. It shows up the TrueCrypt screen, but after typing in the password it crashes the system. When I run my little utlility (w00t.com) on a normal WinXP machine it seams to crash immedealty.
Apparently I'm forgetting some crucial stuff the BIOS normally does, my guess is it's something trivial. Can someone with better bare-metal DOS and BIOS experience help me out?
Heres my code:
.MODEL tiny
.386
_TEXT SEGMENT USE16
INCLUDE BootDefs.i
ORG 100h
start:
; http://vxheavens.com/lib/vbw05.html
; Before DOS has booted the BIOS stores the amount of usable lower memory
; in a word located at 0:413h in memory. We going to erase this value because
; we have booted dos before loading the bootsector, and dos is fat (and ugly).
; fake free memory
;push ds
;push 0
;pop ds
;mov ax, TC_BOOT_LOADER_SEGMENT / 1024 * 16 + TC_BOOT_MEMORY_REQUIRED
;mov word ptr ds:[413h], ax ;ax = memory in K
;pop ds
;lea si, memory_patched_msg
;call print
;mov ax, cs
mov ax, 0
mov es, ax
; read first sector to es:7c00h (== cs:7c00)
mov dl, 80h
mov cl, 1
mov al, 1
mov bx, 7c00h ;load sector to es:bx
call read_sectors
lea si, mbr_loaded_msg
call print
lea si, jmp_to_mbr_msg
call print
;Set BIOS default values in environment
cli
mov dl, 80h ;(drive C)
xor ax, ax
mov ds, ax
mov es, ax
mov ss, ax
mov sp, 0ffffh
sti
push es
push 7c00h
retf ;Jump to MBR code at 0:7c00h
; Print string
print:
xor bx, bx
mov ah, 0eh
cld
@@: lodsb
test al, al
jz print_end
int 10h
jmp @B
print_end:
ret
; Read sectors of the first cylinder
read_sectors:
mov ch, 0 ; Cylinder
mov dh, 0 ; Head
; DL = drive number passed from BIOS
mov ah, 2
int 13h
jnc read_ok
lea si, disk_error_msg
call print
read_ok:
ret
memory_patched_msg db 'Memory patched', 13, 10, 7, 0
mbr_loaded_msg db 'MBR loaded', 13, 10, 7, 0
jmp_to_mbr_msg db 'Jumping to MBR code', 13, 10, 7, 0
disk_error_msg db 'Disk error', 13, 10, 7, 0
_TEXT ENDS
END start
